1. Field of the Invention
The present invention relates to a CDMA (Code Division Multiple Access) radio transmission system used for mobile communication, or the like, and particularly to a CDMA receiving apparatus in which coherent detection performance in multi-code transmission is improved by a hardware configuration which can be produced easily.
2. Description of the Related Art
In CDMA system mobile communication, spread-spectrum communication, in which the spectrum of an information signal to be transmitted is spread by using a spread code to a sufficiently wide band compared with the original information band width, is performed, so that multiple access based on the allocation of the spread code to users, that is, code division multiple access is performed.
Further, in digital communication, it is known that coherent detection using the same carrier frequency both in transmission and in reception to perform modulation and demodulation of a signal has more excellent error-rate characteristic than differential detection for obtaining the phase difference in reception wave at a distance of one symbol as a detection system.
In the coherent detection, however, if there is fading, the error-rate characteristic is deteriorated by the phase change after reception. In the case of PSK (phase shift keying) modulation, demodulation data may be rotated on a phase plane due to the phase change or may occupy a phase point different from the original phase point.
In order to solve the problem in such coherent detection, there is used a system in which the transmission side transmits a known signal (pilot signal) contained in transmission data to the reception side so that the reception side estimates the condition of line on the basis of the result of reception of the pilot signal and corrects the distortion of the transmission signal due to fading.
As shown in FIG. 8, a CDMA transmitting apparatus using this system comprises: a switch 86 for outputting a transmission data series or a pilot signal selectively; a spread code generator 83 for generating a spread code to be allocated; a spreader 82 for spreading data to be transmitted by using the spread code; a transmission RF section 87 for up-converting transmission information into a radio-frequency signal; and an antenna 88 for transmission.
In this transmitting apparatus, when the switch 86 connected to the lower side is changed over to the upper side at regular intervals, the pilot signal is inserted into the transmission data series at regular intervals. The transmission information containing the pilot signal inserted therein is spread by the spreader 82 on the basis of a spread code of the code #0 generated by the spread code generator 83 and is up-converted into a radio-frequency signal by the transmission RF section 87, so that the radio-frequency signal is transmitted through the antenna 88.
In the reception side, after the frequency of the reception signal is down-converted, the reception signal is despread on the basis of the spread code of the code #0 and, further, the channel condition is estimated on the basis of the relation between the result of the demodulation of the pilot signal inserted in the reception signal at regular intervals and the original phase position of the pilot signal. While correction is performed on the basis of the result of the estimation, coherent detection of data continued to the pilot signal is performed.
According to this system, because the pilot signal is inserted into the reception data series at regular intervals, correct demodulation can be performed regardless of the time change of the transmission condition due to the fading.
The article "Performance of Coherent RAKE Detection using Interpolation on DS/CDMA" by Higashi et al., described in IEICE Technical Report, October 1994, has proposed a decision feedback interpolation type coherent detection system using a small number of pilot symbols to improve accuracy in estimation of a channel transmission function in DS/CDMA mobile radio communication and to make it possible to follow high-speed fading. According to this system, a pilot symbol is periodically inserted in an information symbol to be transmitted to thereby estimate the transmission function of the transmission system on the basis of both decision feedback and interpolation, so that the error rate can be reduced.
Further, in CDMA, a system of "multi-code transmission" in which a plurality of channels, that is, a plurality of spread codes are allocated to one user and information data is divided into a plurality of channels to be transmitted is known as a system for transmitting information over an information transmission speed per channel (spread code).
Also in the case of multi-code transmission, use of the pilot signal has been proposed to improve the coherent detection performance of each channel. There has been discussed a system in which a pilot channel for transmitting only a pilot signal is provided newly or the pilot signal is transmitted while being shifted in time for every channel, and so on.